Innovative Thermal Oxidizer Design Reduces
Emissions and Natural Gas Consumption
By Clark Griffith, PE

Al-Corn Clean Fuel of Claremont, Minnesota, faced what dozens of corn-to-ethanol plants have faced over the last three years. It had to meet environmental requirements for carbon monoxide (CO), volatile organic compounds (VOCs), and nitrogen oxides (NOx) for the distillers grains dryer flue gas it was emitting to the atmosphere. What Al-Corn faced is also similar to what many other ethanol plants faced in that Al-Corn already had installed equipment designed to meet its emissions requirements. Unfortunately, the installed thermal oxidizer (T.O.) with heat recovery steam generator (HRSG) was not destroying the CO and VOCs and limiting NOx generation to the levels required by state "minor source emission" standards. In addition, the marginal VOC and CO performance was at temperatures that were too high to match the facility's steam requirements.

Al-Corn hired Met-Pro Systems Division to assess the problems with their T.O. and recommend changes to meet the emission applicable levels while minimizing natural gas consumption. Ultimately, Met-Pro implemented the fuel savings changes and the modifications to Al-Corn's T.O. required to meet the emissions limits.
Engineering Study
The first step was to assess the root cause of the Al-Corn T.O.'s inability to completely destroy organics without higher than desired operating temperatures. Based  on  the  detailed assessment of the Al-Corn T.O.,  it was clear  that  Al-Corn's existing T.O. would have to be modified to achieve environmental compliance. Important to the economic practicality of modifying the T.O. was that the facility's HRSG produce no more than the required amount of steam to operate the plant.

Computer Modeling
Of paramount importance in designing thermal oxidizers is mixing of the incoming fume with the flame¹s hot flue gases. Data clearly indicate that the reaction of CO in thermal oxidation is very rapid once the CO-containing gas is well mixed (This assumes the presence of at least several percent oxygen and adequate temperature). To determine how well the gases were being mixed in the T.O., computational fluid dynamics (CFD) was utilized. CFD is a tool that capitalizes on high computer processing speeds allowing for the evaluation of fluid flow in process equipment. CFD effectively reduces large volumes, such as in a T.O., into smaller discrete cells in which local flow, temperature, and compositional conditions can be calculated and meshed with other local cells until the entire T.O. model is converged.

Al-Corn¹s down-fired T.O. with incoming dryer flue gas and internal geometry were loaded into the CFD model. The model indicated that the cold dryer flue gas that entered tangentially at the top of the T.O. was not mixing adequately with the hot burner products of combustion.

To improve gas mixing, several changes were implemented based on the CFD model. The first was to reduce the size of the T.O.'s natural gas burner from 210MM Btu/hr capacity to 160MM Btu/hr capacity. This change would increase the flame velocity significantly and induce better gas mixing.

To inject the dryer flue gas into the T.O., a ring plenum with multiple injection points was installed. The plenum provided four primary dryer flue gas injection points downstream of the burner flame. Waiting until the flame is fully developed to inject the majority of the dryer flue gas minimizes the potential for CO creation that can occur when the flame is disrupted or quenched. The ring plenum also allowed for ten additional dryer flue gas injection points to be installed on the burner face. This allowed a portion of the dryer flue gases to mix with the flame near its base, while keeping the flame zone cool to help minimize NOx formation.

"The CO and NOx emissions performance of the reconstructed thermal oxidizer at Al-Corn far exceeds the applicable performance criteria. The VOC emissions performance was as good as can be demonstrated by typical stack testing procedures at over 99.8%. Achieving the demonstrated emissions performance at operating temperatures below 1440°F is outstancing and may be unique in the ethanol industry", said Todd Potas, PE, QEP, of Natural Resource Group, Inc.

Heat and Material Balance
A heat and material balance was performed for Al-Corn's expected operating conditions that also highlighted a number of shortcomings of the existing T.O. The most obvious outcome of the heat and material balance was that the T.O. was considerably short on residence time. Typically, a T.O. designed to eliminate 90% of CO at 1500°F would have at least 1 second of flue gas residence time. Al-Corn's T.O. flue gas had, at best, 0.4 seconds residence time from the end of the flame to the end of the oxidizer chamber.

It was evident that the T.O. would have to be extended in some way to add residence time. After considering various alternatives, it was determined that the best way to add chamber volume to the T.O. was to install an up-fired chamber that would horizontally connect through a duct to the top of the existing T.O. chamber. This design had several benefits. Burner access would be at ground level instead of 40 feet in the air. Also, the 180° turn the T.O. would take would aid mixing of the flue gas. Finally, using the up-and-over design also took full advantage of the existing T.O. chamber¹s residence time.

Finally, and key to the economics of the T.O. revamp, the heat and material balance indicated that at the projected 1500°F operating temperature the T.O.¹s waste heat boiler (WHB) would produce 20-30% more steam than the plant needed to operate. Venting the excess steam equated to roughly $1MM per year in fuel consumption.

To allow the T.O./WHB to operate at sufficient temperature to meet emissions requirements while producing only the steam required by the plant, a recuperative gas-to- gas heat exchanger was installed. This efficient heat exchanger takes heat from the T.O. flue gases and to preheat incoming 205°F dryer flue gases to as high as 955°F. The unique control scheme allows for maximum heat recovery with precise steam production to meet the plant¹s requirements.

Table 1 - Al Corn Clean Fuel Dryer/T.O. Emissions Limits and Actual Emissions
Emissions Criterion
Minor Source Limit
Consent Decree Limit
Actual Emissions
CO
100 tpy
70 tpy
15.8 lb/hr
62.8 tpy (55 ppmv)
14.6 lb/hr
VOCs
95% destruction
100 tpy

95% destruction
1.5b/hr

99.8% destruction
(1 ppmv)
0.11 lb/hr
NOx
100 tpy
20.5 tpy
0.040 lb/MM Btu
16.4 tpy
(9 ppmv)
0.11 lb/MM Btu

NOx Control
Due to an EPA requirement for best available control technology (BACT), Al-Corn had a very restrictive NOx limitation compared with other ethanol plants with a T.O. not required to install BACT. As previous tests had shown as high as 73 tpy NOx from the combined dryer/T.O. stack, the required 30 tpy NOx limit for the two dryers and the T.O. was going to require significant changes to the combustion systems already installed at Al-Corn.

It was decided that flue gas recirculation (FGR) would be installed on each of the two dryer combustion systems as well as on the T.O. FGR mixes flue gases from the given combustion chamber with the combustion air to the burner. This mixture has a lower oxygen concentration and higher moisture concentration than ambient air. With such a mixture, the burner flame temperature is reduced, which causes a decrease in the level of (thermal) NOx produced.

Emissions Results
Official stack tests on the revamped Al-Corn T.O. were performed in August. The tests were to demonstrate that the combined DDGS dryer/T.O. stack met the emissions limits set for CO, VOCs, and NOx. Results are presented in the table below and were achieved at an average operating temperature of 1436°F.

Sound T.O. design principles have been lacking in the corn-to-ethanol industry to date. This has led to higher operating temperatures, excessive fuel and power consumption, equipment replacement and repair ­ not to mention plant downtime and lost ethanol production.

Randy Doyal, CEO of Al-Corn, emphasizes another positive aspect of the T.O. project, "With a dozen or more thermal oxidizers installed in ethanol plants in the U.S., the T.O. operating temperature range has been 1530-1610°F. Because we're able to operate our T.O. at 1436°F, we'll save $500,000 a year in natural gas."

The necessary T.O. design principles include, first and foremost, thorough mixing of the dryer flue gas with burner flue gas. Secondly, adequate residence time is required to allow for sufficient destruction of CO and VOCs. Once these first two principles are adequately addressed, NOx minimization is more easily addressed because the lowest possible T.O. operating temperature is achieved, which contributes to lower NOx generation. At that point, low NOx burners and/or flue gas recirculation can be used to achieve very low NOx levels.

Randy Doyal, CEO of Al-Corn, emphasizes another positive aspect of the T.O. project, "With a dozen or more thermal oxidizers installed in ethanol plants in the U.S., the T.O. operating temperature range has been 1530-1610°F. Because we're able to operate our T.O. at 1436°F, we'll save $500,000 a year in natural gas."







Clark Griffith, PE, is General Manager of Met-Pro Systems.
Reach him by e-mail at cgriffith@met-pro.com or by phone at 215-723-9300 x 275.